Accurate control of a synchronous rectification switch is difficult to achieve where zero crossing point detection of the resonant current through the switch is required. This includes accurate control of a synchronous rectification switch on the secondary side of an LLC converter, where accurate zero crossing point detection of the resonant current through the secondary side synchronous rectification switch directly affects efficiency.
FIG. 1 illustrates an exemplary synchronous rectification switch implemented as a MOSFET Q1. Due to the inductance (Lpkg) of the package in which the MOSFET Ts is included, the drain-source voltage dd′s′ measurable at the package terminals d′, s′ has a leading phase difference with respect to the current iSR through the MOSFET Ts. The package is graphically illustrated as a dashed box in FIG. 1. If the sensed dd′s′ voltage is used to detect the zero crossing point of the resonant current ISR, the MOSFET Ts will be turned off while the device is still conducting current. This switching condition leads to poor efficiency.
FIG. 2 illustrates this problem in greater detail, where Vd′s′ is the drain-source voltage measured at the package terminals d′, s′, iSR is the resonant current through the MOSFET Ts, Vds is the actual MOSFET drain-source voltage, VgsSR is the control voltage applied to the gate of the MOSFET Ts, Vth corresponds to the threshold voltage of the MOSFET Ts, ton is the actual time the MOSFET Ts is on, and tθ represents the variability in the switch off period due to the phase difference between dd′s′ and ISR. At a fixed frequency, the phase difference between Vd′s′ and ISR is fixed and therefore may be compensated easily. However, when the switching frequency changes, the phase difference also changes proportionally which further complicates the zero crossing point detection. This is illustrated in FIG. 2 by the two different Vd′s′ curves (a) and (b), each one of which corresponds to a different switching frequency.
Several methods have been proposed to address accurate zero crossing point detection. These methods involve complex sensing circuitry or control algorithms. A simple and accurate solution is therefore desirable.